In Raman spectroscopy, a laser beam is directed onto a sample and the radiation reflected from the sample is collected, passed through an interferometer, and then detected. The reflected radiation is characterized by a strong component, known as the Rayleigh line, at the wavelength of the laser, with weaker secondary components, known as the "Stokes" and "anti-Stokes" lines, at wavelengths slightly above and below the laser wavelength. The Stokes and anti-Stokes lines are referred to collectively as the "Raman" lines. The objective in Raman spectroscopy is to analyze the Stokes and/or anti-Stokes lines for spectral information about the sample.
The Raman scattering effect is illustrated in FIG. 1. For example, using an Nd:YAG laser as the excitation source, the sample is raised to an excitation level equal to that of the source and it then decays in one of three possible ways. It can fall back to its original ground state, to the next higher energy state, or, if the original state was the first electronic state as expected by Boltzman's distribution, from the first electronic state to the ground state. These three possibilities result in the Rayleigh scattering, Stokes scattering, and anti-Stokes scattering, respectively.
A difficulty encountered in Raman spectroscopy is separating the relatively weak signals of the Raman lines from the relatively strong signal of the Rayleigh line. The Rayleigh line is usually very close to the Raman lines, and has a much greater intensity (typically, two to eight orders of magnitude greater) than the Raman line intensities. Because the strong Rayleigh line is very close to the weak Raman lines, the Rayleigh line will tend to swamp the Raman lines and limit the dynamic range of detection. Although electronic filtering may be utilized to reduce the effect of the Rayleigh line in the electrical signal after the beam is detected, this approach is usually not satisfactory because most detectors are severely disturbed by the large Rayleigh line component. Typically, elimination of the Rayleigh line in Raman spectrometers involves optical filtering of the scattered radiation to attenuate the Rayleigh line while leaving the Raman lines substantially unattenuated.
A typical prior art Raman spectrometer arrangement is shown in FIG. 2. The light beam 20 from a laser 21 is reflected by mirrors 22 and 23 to sample collection optics 25 which allows the beam 20 to strike the sample 27 and then collects the reflected radiation and collimates it into a beam 29. The beam of reflected radiation 29 is passed through a stack of filters 30 before the beam proceeds into an interferometer 32 (e.g., a Michelson interferometer shown schematically as having a beam splitter 34, a moving mirror 35 and a stationary mirror 36). After passing through the interferometer, the beam 37 is collected by optics shown schematically at 39 and focused onto a detector 40. To achieve sufficient attenuation of the Rayleigh line, the stack of filters 30, typically dielectric filters, produce attenuation of the Rayleigh line on the order of 10.sup.-6. Dielectric filters are conventionally edge filters, which attenuate either the Stokes or anti-Stokes line as well as the Rayleigh line. A disadvantage of the use of a filter stack is that the throughput of the passband of the filter is reduced by each additional filter. A throughput of only 30% of the desired Raman wavelengths would be typical for a five filter stack typically required to achieve 10.sup.-6 attenuation of the Rayleigh line.